The present invention relates to a current controlling device and a signal converting apparatus applying the current controlling device, and more particularly to a current controlling device having constant input impedance.
For a functional circuit, gain is a measure of ability to increase the power or amplitude of a signal from the input to the output. The gain of the functional circuit may need to be adjusted in order to adapt with the system in which the functional circuit is installed therein. Therefore, how to control the gain of a functional circuit is an important issue in the field of circuit design. For the example of a trans-impedance amplifier (TIA), in which the trans-impedance amplifier is a feedback system comprising an amplifier, a feedback resistor, and a feedback capacitor, the gain of the trans-impedance amplifier is decided by the impedance of the feedback resistor. Therefore, the gain of the trans-impedance amplifier can be adjusted by the adjustment of the feedback resistor. However, this may vary the bandwidth of the trans-impedance amplifier. To keep the bandwidth intact, the capacitance of the feedback capacitor may also need to be adjusted accordingly. In other words, this conventional way will increase the complexity of controlling the trans-impedance amplifier.
Another way to control the gain of a functional circuit is to adjust the supply current of the functional circuit. For the example of a current mode DAC (Digital-to-analog converter), the gain of the current mode DAC is decided by the supply current or tail current of the current mode DAC. However, when the supply current or tail current of the current mode DAC is adjusted, the matching impedance of the current mode DAC will also vary. In other words, this conventional way is difficult to meet the matching requirement of the following circuit connected to the current mode DAC especially when the tail current is small.
Therefore, providing an effective way to control gain of a functional circuit is an important issue in the field of circuit design.